Electrocyclic reactions definition

A third and critical advance in the development of the Nazarov cyclization was the demtmstration that it belongs to the general class of cationic electrocyclic reactions (Scheme 4). This broadened its definition to include reactions which involve pentadienylic cations or equivalents and thus expanded the range of precursors for cyclopentenones. Further, the stereochemical features of electrocyclization enhanced the utility of the reaction and, in addition, stimulated the development of a photochemical variant. 

With these descriptors in hand, we can look at the generalized orbital symmetry rule. There is a definite binary nature to the theory of pericyclic reactions. For cycloadditions, [2-f2] is forbidden (all suprafacial), whereas [4-F2] is allowed (all suprafacial). Continuing with the series, [6+2] is forbidden, and [8+2] is allowed. We will also encounter patterns in the other kinds of pericyclic reactions presented electrocyclic reactions, sigmatropic shifts, etc. Based on patterns such as these. Woodward and Hoffmann proposed the following rule for all pericyclic reactions ... 

Pericyclic reactions were defined in 1969 by R. B. Woodward and R. Hoffmann as reactions in which all first-order change.s in bonding relationships take place in concert on a closed curve that is, as concerted reactions in which all bonds are formed or brokeasimultaneously around a circle. This definition arose from the systematic study of the conservation of orbital symmetry in a series of reactions electrocyclic reactions, cycloadditions, sigmatropic shifts, cheletropic reactions, and group transfer and elimination reactions. Excellent reviews on the historical evolution of pericyclic reactions have been published. ... 

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